In the recent technique for removal of ionic substances contained in seawater, brackish water or the like, separation methods utilizing separation membrane elements have found increasing uses as processes for energy savings and conservation of resources. Separation membranes adopted in the separation methods utilizing separation membrane elements are classified into five groups according to their pore sizes and separation performance, namely microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, reverse osmosis membranes and forward osmosis membranes. These membranes have been used in e.g. production of drinkable water from seawater, brackish water or water containing deleterious substances, production of ultrapure water for industrial uses, effluent treatment, recovery of valuable substances, or the like, and membranes to be used therein have been changed to suit ingredients targeted for separation and separation performance requirements.
Separation membrane elements can have a wide variety of shapes, but they have commonality in the sense that water to be treated (raw water) is fed to one surface of a separation membrane and a permeated fluid is obtained from the other surface of the separation membrane. By having a plurality of separation membranes tied in a bundle, each separation membrane element is configured to extend the membrane area per separation membrane element, or equivalently, to increase the amount of a permeated fluid obtained per separation membrane element. Various types of shapes, such as those of a spiral type, a hollow fiber type, a plate-and-frame type, a rotating flat-membrane type and flat-membrane integration type, have been proposed for separation membrane elements in keeping with their intended uses and purposes.
For example, spiral-type separation membrane elements have been widely used in reverse osmosis filtration. Each spiral-type separation membrane element is provided with a central tube and a stack wound up around the central tube. The stack is formed by stacking a feed-side channel member for feeding raw water (that is, water to be treated) to the surface of a separation membrane, a separation membrane for separating ingredients contained in the raw water and a permeate-side channel member for leading, into the central tube, a permeate-side fluid having been separated from the feed-side fluid by passing through the separation membrane. In the spiral-type separation membrane element, it is possible to apply pressure to raw water, whereby it becomes possible to take out a permeated fluid in greater quantity. In this respect, the use of spiral-type one is advantageous.
In the spiral-type separation membrane element, a net made mainly from a polymer compound has been generally used as the feed-side channel member in order to form a flow channel for a feed-side fluid. In addition, a multilayer-type separation membrane has been used as the separation membrane. The multilayer-type separation membrane is a separation membrane including a separation functional layer (porous supporting layer) formed from a cross-linked polymer compound such as polyamide, a porous resin layer formed from a polymer compound such as polysulfone, and a nonwoven fabric substrate made from a polymer compound such as polyethylene terephthalate, in which these layers are stacked in the order mentioned, from the feed side toward the permeate side. And as a permeate-side channel member, a knitting member referred to as tricot finer in mesh than the feed-side channel member has been used for the purposes of preventing the separation membrane from sinking and of forming a permeate-side flow channel.
As demands for reduction in cost of fresh water production have grown in recent years, membrane elements having higher performance have been required. For example, with the intention of improving separation performance of separation membrane elements and increasing the amount of permeated fluid per unit time, improvements in performance of separation membrane element members, such as channel members, have been suggested.
Specifically, a separation membrane element having a sheet embossed with an uneven pattern as a permeate-side channel member has been proposed (see Patent Document 1). Also a separation membrane element requiring neither a feed-side channel member such as net nor a permeate-side channel member such as tricot has been proposed by arranging a channel member formed of an elastomer called a vane in the separation membrane (see Patent Document 2). Further, a separation membrane element having a channel member with yarns arranged on a nonwoven fabric, has been proposed (see Patent Document 3).